The present invention relates to a method for determining the flow of a fluid flowing in a pipe by measuring the temperature of the fluid and the time needed for a probe, which is brought to a first temperature different from that of the fluid, to be brought to a second temperature closer to that of the fluid.
Various solutions have already been proposed for determining the flow of a fluid in a pipe on the basis of the heat exchange between a probe and the fluid. In general, this involves using a probe of which the variation of electrical resistance is measured as a function of temperature. This is a principle based on that of the hot-wire anemometer.
This kind of flow meter is described notably in the GB Pat. No. 1 345 324. According to this patent, the heating power of the probe is supplied by a series of identical impulses separated by intervals which are sufficiently long to ensure that the probe has returned to its temperature of equilibrium with the fluid at the beginning of each impulse. The variations in flow are detected by variations recorded from one impulse to the next in the rate of change of electrical resistance in the probe during an impulse. The rate of change of resistance is given by the variation of voltage derived from a thermistor immersed in the fluid. The voltage is applied to a voltage-frequency converter, the frequency of which diminishes continuously in proportion to the progressive reduction of voltage. Then, the frequency arising during the duration of the impulse is measured and the flow calculated.
From the electronic point of view, the majority of the signal processing is analog, which leads to a relatively complex, and hence a relatively costly, electronic circuit. Moreover, the transformation of voltage to variable frequency covers a rather narrow range of measurements. This is equally true as regards impulses for heating with a constant strength.
A simplification has been proposed, comprising measuring a time difference between two determined temperature thresholds and then calculating the flow with the aid of exponential equations. Such solutions are described notably in U.S. Pat. No. 3,498,128 and EP-Al No. 0070 801.
Apart from the fact that the solving of exponential equations is, from the electronic point of view, still a relatively complex solution, it has been shown that the exponential cooling curves of the probe vary not only according to the flow of the fluid, but also in relation to the temperature of the fluid, hence the need to take temperature into account when solving these equations.
Conventional thermal energy consumption meters measure, on the one hand, the difference in temperature between the inlet and the outlet of the heating apparatus and, on the other hand, the flow by a conventional water flowmeter, for example a turbine flowmeter. In this kind of apparatus, although the precision of the flow measurement is satisfactory at high flow rates, it becomes mediocre at low flow rates (below 10-20% of the nominal flow) which are the most frequent in central heating installations.
There is a market in the field of meters for the consumption of thermal energy distributed in the form of a fluid, notably in the field of hot water consumption meters of an installation for collective central heating or for distributing hot water for sanitation. Again, the price of the meter has to be acceptable, i.e. capable of being paid off in a few years with the savings which will be made possible by such an installation, consequent on the effective distribution of the charges with relation to the energy consumed and not, as previously, in proportion to the volume occupied. This method of charging is an incentive to save in so far as each person is responsible for their own consumption. This method of charging is becoming legally obligatory, at least in new accommodation, in certain countries.
The aim of the present invention is to provide a flow measurement method and apparatus which are both accurate and sufficiently simple to be applicable in the above-mentioned areas, where price and accuracy are two factors which must be compatible, as well as being applicable in industry and to fluids other than liquids, notably steam.